In order to improve safety of a machine, operated by an air pressure actuator, a combined safety valve of high safety is put into practical use, in which safety valves formed of a pilot-type single action three-port valve are doubly installed, and in which the supply of air to an actuator is assuredly enabled to block off even when the safety valve on the other side is normal, in a case that a failure occurs on one safety valve side.
In a patent document 1 (Japanese Unexamined Utility Model Registration Application Publication No. 7-32269) described as an example, a dual safety valve for use in press, in which two sets of main valves (two-position three-port valve) are embedded in parallel in a valve body, and in which a housing where two sets of pilot valves of an electromagnetic-type are embedded is disposed on an upside of the valve body, and in which a position detecting device of the main valve is disposed on a downside of the valve body, is disclosed. The position detecting device of the main valve is configured such that a detecting pin fixed on the two sets of main valve is projected to the downside of the valve body, a magnet moves in an interlocking manner with a movement of the detecting pin, and the movement of the magnet is detected by means of a magnetic sensitivity-type switch.
In the dual safety valve, when both the pilot valves are inoperative, both the main valves are located at an upper position and an operating port is allowed to communicate with a discharge port, and each of the detecting pins of both the main valves is located at an upper position, and thereby both switches are turned off. When both the pilot valves are operated, both the main valves move to a lower position, an intake port and an operation port are allowed to communicate with each other and air is supplied to a press, and each of the detecting pins of both the main valves moves to a lower position, and thereby both the switches are turned on. During the time when both the pilot valves are operated, and when the main valve of one side does not move to the lower position due to a failure thereof, the switch of one main valve is turned off and the switch of the other main valve is turned on. A control circuit detects an erroneous operation and returns the other main valve to the non-operating position so that the air is not supplied to the press.
However, in the dual safety valve in the patent document 1, since two sets of the pilot valves, the main valves, and the position detecting devices of respective main valves are respectively disposed in the single housing, the single valve body, and a single lower chassis, there is a problem that the dual safety valves have to be entirely exchanged even when the failure occurs only on one side of the pilot valves, the main valves, and the position detecting devices.